The present invention relates to a method for measuring the anisotropy of a zone of the surface of an element comprising at least one fissile material, the method being of the type comprising the steps of:                transmitting a beam of light onto the surface and        passing the beam of light reflected by the surface into a polarisation analyser having a modifiable analysis direction.        
The invention is used in particular, but not exclusively, for controlling particles of nuclear fuel for a high temperature nuclear reactor (HTR) or a very high temperature nuclear reactor (VHTR).
Such particles are generally spherical and comprise a fissile core which is surrounded by layers of dense and porous pyrocarbon and silicon carbide.
Those particles are intended to be embedded in graphite matrices in order to be able to be introduced into a reactor. Those matrices are, for example, in the form of ovoids or cylinders, sometimes referred to as compacts.
The quality of the layers of dense pyrocarbon determines the life-span of the particles during their irradiation in the reactor. Under irradiation, pyrocarbon tends to become anisotropic, which brings about a stressed state which may compromise the integrity and uniformity of the particles owing to rupture of the layer of silicon carbide.
At the end of the operation for producing the particles, therefore, their pyrocarbon layers must be as isotropic as possible and it is desirable to be able to control the degree of anisotropy thereof with tools suitable for rapid control of an industrial type.
U.S. Pat. No. 3,972,619 describes a method which allows the anisotropy of the pyrocarbon layers of such particles to be measured. The measurement is carried out on a metallographic section in an equatorial plane of a particle.
A beam of monochromatic polarised light is transmitted onto the sectioned surface of the particle. If the zone of that surface that is illuminated by that beam is not isotropic, it will bring about slight depolarisation of the beam when it is reflected. Rotation of the direction of the polarisation of the incident beam is brought about so that the polarisation direction of the reflected beam oscillates.
The amplitude of the oscillations is established by measuring the amplitude of the oscillations of the intensity detected by a photometer, after the reflected beam has been passed into a polarisation analyser. The analysis direction of the polarisation analyser is modified and measurements of the amplitude of the oscillations are carried out with different analysis directions.
Based on those different measurements, parameters characterising the anisotropy in the zone illuminated by the incident beam are calculated.
Such a method requires a relatively complex and costly installation, in particular because of the presence of the large number of pieces of optical equipment and the photometer. The method is also found to take a long time to carry out.
There have also been envisaged methods for measuring the anisotropy which were not optical methods, but were instead based on a technique involving diffraction of X-rays. However, such methods have been found to be unreliable for this application, in particular owing to the spherical shape of the particles studied.
More recently, U.S. Pat. No. 5,956,147 proposed a method based on ellipsometry. A polarised light beam is transmitted elliptically onto a metallographic section of a particle. The reflected beam then passes into a quartz crystal, then into a polariser, before being directed to a photomultiplier tube, whose output signal is processed in order to extract from it a diattenuation coefficient which is correlated with the anisotropy. Such a method is also costly and complex to carry out.
The problem addressed by the invention is to overcome this problem by providing a method for measuring anisotropy which is reliable, rapid to carry out and requires a less expensive installation.